While liquid crystal displays are in wide use in recent years, liquid crystals per se are not luminescent, so that for use in the dark, the liquid display needs to be provided with a backlight at a rear portion thereof for the user to view presentations with the light passing through the liquid crystal. Cold-cathode discharge tubes having a long life are chiefly used as such backlights. With these cold-cathode discharge tubes, however, initiation of discharge causing stabilized firing due to estabilished discharge after energization is dependent on the number of electrons or cations remaining within the tube. Accordingly, if the amount of electrons or cations present is insufficient, there arises the problem of delayed initiation of discharge. Electrons and cations are produced primarily by external light, so that in the case where the liquid display is used as incorporated in other devices, external light through the liquid crystal is not expectable, and the problem becomes more serious owing to scarcities of electrons and cations.
FIG. 21 shows a discharge tube 1 which is provided with a conductor 2 as is already known to overcome the above drawback ("Shomei Kogaku (Illumination Engineering)," edited by Denki Gakkai, p. 72, published by Ohm Co., Ltd. in Jul., 1963). With reference to the drawing, the discharge tube 1 has a cathode 3 at its one end and an anode 4 at the other end thereof. The conductor 2 is adhered to the outer bottom surface of the tube 1 to extend from below the cathode 3 to below the anode 4. (Such a conductor is positioned close to the cathode and anode and will hereinafter be referred to as a "proximity conductor.") The cathode 3 and the anode 4 are connected via a switch 5 to a d.c. power source 6, the high-voltage side being connected to the anode 4, and the ground side to the cathode 3. The proximity conductor 2 is at the same potential as the cathode 3. The discharge tube 1 has a vacuum inside thereof and has small amounts of mercury and argon gas enclosed therein.
When a high voltage is applied to the anode 4 by closing the switch 5, a high electric field is set up across the anode 4 and a portion of the proximity conductor 2 close to the anode 4 since the conductor 2 has the same potential as the cathode 3, highly accelerating electrons and cations present in very small amounts within the discharge tube 1. While migrating toward the anode 4, the accelerated electrons vigorously collide with mercury atoms. A majority of accelerated cations vigorously collide with the proximity conductor 2. These collisions produce secondary electrons, which further collide with mercury atoms to cause emission of additional secondary electrons. Repeated collisions thus taking place produce rapidly increased quantities of electrons and cations, leading to expedited initiation of discharge. Nevertheless, the conventional method of FIG. 21 is not fully effective; many discharge tubes thus adapted still include some which can not be initiated into operation without a delay.